IOS 12 will not allow Airplay to multiple Apple Airport

I am now trying to Airplay from my iPhone to the HomePod, and 2 Airport Expresses. This works fine from MacOS Mojave. The iPhone will connect to the HomePod, but will only allow one of the two Airport expresses at once. As soon as I try to connect the second one, the first one disconnects, as if there is a "limit" on devices that can connect? Given this all works from the Mac ... does IOS have a bug?